Electronic security systems are known for the detection of the unauthorized removal of items containing a resonant tag circuit. Such systems employ a transmitter providing an electromagnetic field in a zone or region under surveillance, and a receiver operative to detect a resonant tag frequency caused by the presence of a tag in the surveillance zone and to provide an output alarm indication of tag presence. A preferred electronic security system is described in U.S. Pat. Nos. 3,810,147; 3,863,244, and 3,967,161.
In electronic security systems such as those described in the above-cited patents, two identical planar single loop antennas are usually employed, one for transmitting and one for receiving. The transmitting loop antenna generates an electromagnetic field which extends far beyond the immediate area of the security system necessary for system operation. In addition, the receiving antenna is sensitive to external noise generated at great distances from the receiver relative to the small area of interest of system operation.
An antenna system is described in U.S. Pat. No. 4,016,553 in which the inherent problems of a simple loop antenna in an electronic security system are minimized by use of two or more identical parallel loop antennas connected in phase opposition or bucking relationship. The antenna system comprises a cluster of at least two parallel electrically conductive loops of similar size connected in phase opposition so that current always flows in mutually opposite directions through corresponding portions of each loop. As a result, the loops are magnetically arranged in a bucking relationship. The length of and spacing between the loops is small compared to the wavelength of the transmitted or received signals and is disclosed to be typically one tenth of the wavelength. The spacing between the parallel loops is an appreciable fraction, for example one fourth, of the width of the egress passage through which a detectable resonant circuit must pass in a security installation. A separate antenna cluster composed of phase opposed parallel loops can be connected to respective transmitter and receiver of the system, or a single antenna cluster can be employed with both the transmitter and receiver. At distances large compared to the dimensions of the transmitting antenna, the generated electromagnetic waves are cancelled by reason of the phase opposed loop connection. At short distances between the receiving and transmitting antennas, the signals in adjacent parallel antenna conductors do not cancel, resulting in a net detectable signal. Electromagnetic waves incident on the receiving antenna from distances large compared to the antenna dimensions do not provide a sensible antenna signal, but electromagnetic waves incident upon the receiving antenna from sources close to the antenna are sensed to provide a receiving antenna signal.
Thus the antenna system described in U.S. Pat. No. 4,016,553 provides an electromagnetic field in an interrogation region while preventing high intensity fields from occuring outside of the interrogation region. This antenna system also provides detection of selected electromagnetic fields originating in the interrogation region from a resonant circuit while avoiding detection of fields originating from outside of the interrogation region.
The antenna system described in the aforesaid U.S. Pat. No. 4,016,553 suffers several disadvantages in practice. The bucking loop antennas must be separated by a significant distance relative to the distance between the transmitting antenna cluster and receiving antenna cluster. Moreover, the bucking loop antennas must be carefully aligned and balanced for optimum effect. The loops of an antenna cluster are typically spaced apart from each other by a distance corresponding to one fourth the distance across the egress passage. The size of the antenna cluster can become cumbersome for passage widths of conveniently large dimension. For example, for a passage width of six feet, the antenna cluster must be sufficiently large to accommodate a loop spacing of eighteen inches.